Particles able to absorb or scatter light of well-defined colors have been used in applications involving detection, absorption, or scattering of light, for example medical diagnostic imaging. Such particles are typically colloidal metal particles. The term colloidal conventionally refers to the size of the particles, generally denoting particles having a size between about 1 nanometer and about 1 micron.
Small particles made from certain metals that are in the size range of colloidal metal particles tend to have a particularly strong interaction with light, termed a resonance, with a maximum at a well-defined wavelength. Such metals include gold, silver, platinum, and, to a lesser extent, others of the transition metals. Light at the resonance wavelength excites particular collective modes of electrons, termed plasma modes, in the metal. Hence the resonance is termed the plasmon resonance.
By selecting the metal material of a colloidal particle, it possible to vary the wavelength of the plasmon resonance. When the plasmon resonance involves the absorption of light, this gives a solution of absorbing particles a well-defined color, since color depends on the wavelength of light that is absorbed. Solid gold colloidal particles have a characteristic absorption with a maximum at 500–530 nanometers, giving a solution of these particles a characteristic red color. The small variation in the wavelength results from a particle size dependence of the plasmon resonance. Alternatively, solid silver colloidal particles have a characteristic absorption with a maximum at 390–420 nanometers, giving a solution of these particles a characteristic yellow color.
Using small particles of various metals, particles can be made that exhibit absorption or scattering of selected characteristic colors across a visible spectrum. However, a solid metal colloidal particle absorbing in the infrared is not known. Optical extinction, in particular absorption or scattering, in the infrared is desirable for imaging methods that operate in the infrared. Further, optical communications, such as long distance phone service that is transmitted over optical fibers, operate in the infrared.
It has been speculated since the 1950's that it would be theoretically possible to shift the plasmon resonance of a metal to longer wavelengths by forming a shell of that metal around a core particle made of a different material. In particular, the full calculation of scattering from a sphere of arbitrary material was solved by Mie, as described in G. Mie, Ann. Phys. 24, 377 (1908). This solution was extended to concentric spheres of different materials, using simplifying assumptions regarding the dielectric properties of the materials, by Aden and Kerker, as described in A. L. Aden and M. Kerker, J. of Applied Physics, 22, 10, 1242 (1951). The wavelength of the plasmon resonance would depend on the ratio of the thickness of the metal shell to the size, such as diameter of a sphere, of the core. In this manner, the plasmon resonance would be geometrically tunable, such as by varying the thickness of the shell layer. A disadvantage of this approach was its reliance on bulk dielectric properties of the materials. Thus, thin metal shells, with a thickness less than the mean free path of electrons in the shell, were not described.
Despite the theoretical speculation, early efforts to confirm tunability of the plasmon resonance were unsuccessful due to the inability to make a particle having a metal shell on a dielectric core with sufficient precision so as to have well-defined geometrical properties. In these earlier methods, it was difficult to achieve one or both of monodispersity of the dielectric core and a well-defined controllable thickness of a metal shell, both desirable properties for tuning the plasmon resonance. Thus, attempts to produce particles having a plasmon resonance in keeping with theoretical predictions tended to result instead in solutions of those particles having broad, ill-defined absorption spectra. In many instances this was because the methods of making the particles failed to produce smooth uniform metal shells.
Further, an anticipated difficulty in application of particles having metal shells is their susceptibility to loss of structure under heating. Thus, methods of protecting particles having metal shells are desired. Still further, due to material and size constraints, it is anticipated to be difficult to extend the plasmon resonance of a particle with a single shell into the mid and far infrared.
Thus there remains a need for improved plasmon resonant particles and methods of making them.